For a rubber composition for tread of pneumatic tires, in particular a rubber composition for tread of high performance tires such as competition tires, there is a strong demand of high grip performance from the beginning of running (initial grip performance) and stable grip performance during running (grip performance during running), and various measures have been taken to secure the both performance.
In order to improve initial grip performance, a method of using a polymer with a low styrene content, a low softening point resin or a softener with a low temperature for a rubber composition for tread has been considered. On the other hand, in order to stabilize grip performance during running, a method of using a polymer with a high styrene content or a high softening point resin for a rubber composition for tread has been considered.
However, in tires having a tread compounding a polymer with a low styrene content or a low softening point resin, while initial grip performance is improved, there is a problem that grip performance during running is lowered as a temperature of the tread increases. On the other hand, in tires having a tread compounding a polymer with a high styrene content or a high softening point resin, there is a problem that initial grip performance and abrasion resistance are significantly deteriorated while grip performance during running is stable.
While JP 10-204216 A discloses a rubber composition for tread of high performance tires that comprises a dipentene-aromatic vinyl copolymer having a specific softening point based on 100 parts by weight of rubber mainly composed of an SBR having a high glass transition temperature, improvement of both initial grip performance and grip performance during running is not considered.